Parametric Analysis of Factors That Affect Asphalt Binder Foaming Characteristics

The use of foamed asphalt binder to produce warm-mix asphalt has increased significantly during the past few years. It is recognized that (1) foaming characteristics of the binder influence the workability of the asphalt mix, and (2) different asphalt binders tend to foam differently with all other factors being the same. The causes for the latter are relatively less explored and is the focus of this paper. The research reproted in this paper investigated the influence of the properties of asphalt binder on its foaming characteristics. A physical model for expansion of asphalt binder foam based on foam physics was used to quantify the effect of water content, temperature, surface tension, and liquid additive (through its effect on surface tension) on the expansion ratio of asphalt binder foam. Analysis of results demonstrates that the bubble size distribution (up to a certain limit) does not directly dictate the maximum expansion ratio of the asphalt binder foam. The analysis also shows that only a fraction of the water added to the binder is effective in creating the foam. The effective water content was strongly correlated with the surface tension of the binder, which was also substantiated by the solubility theory. Surface tension of the asphalt binder was also found to be strongly correlated to the stability of the foam. Foam characteristics of binders modified with additives also demonstrated that stability and expansion of foams can be significantly improved using carefully selected liquid additives.